This grave-maintaining robot was conceptualized by Itamar Shimshony and built by Zvika Markfeld:

As per Jewish customs, it places stones and flowers on the headstone, and occasionally cleans the grave with water and a cloth while patrolling around it. Hardware is based on an Arduino Mega 1280 sitting on top of a Roomba, and connected to a robotic Lynxmotion AL5D arm, a water pump, ultrasonic proximity sensor and the Roomba itself, of course. Most of the challenge was in getting the navigation around the grave right, since as I was loading the Roomba with more and more weight, its internal distance monitoring got skewed until it was rendered unusable. Eventually I came up with this combination of the bump sensors and the ultrasonic rangefinder, as I’m sure you can reverse engineer just by looking at the movement pattern. Other ideas that didn’t make it to the finish line were adding a video projector for the robot to show clips on the headstone itself, including those of itself while browsing Google and looking at mourning customs and dealing with post-traumatic disorders. Also, we thought of making the robot gradually loose its logic and sense of target during the day or the entire period, start running in circles (or whatever verb used to describe a moving Roomba), placing stones and flowers in the thin air, but eventually decided to drop it.